Navigational instruments find use in a wide range of applications such as personal or vehicular navigation and mapping systems, missile guidance systems, target location systems, ships and other vessel's compass systems and the like. One such navigational instrument is an electronic compass, such as a gyro-compass.
A problem with these compass systems is determining the direction of True North in a way that is sufficiently compact, accurate and cost effective enough to be deployed in common applications such as navigation or target location equipment in vehicle and man-portable contexts. Existing gyro compass technologies that are capable of acceptable accuracy are generally too large or expensive.
A typical gyro-compass uses an electrically powered, fast spinning member to determine the direction of True North. A particular gyro-compass is the orbiting accelerometer gyro. Such accelerometers would be suitable for general usage if it were not for the exceptionally high rotor speed required to overcome gravitational sensitivity due to drift in accelerometer alignment.
An orbiting accelerometer gyro compass comprises one or more accelerometers mounted on a spinning rotor with the sensitive axis of each accelerometer nominally aligned parallel to the rotor axis about which it orbits. The spinning accelerometers are subject to alternating Coriolis acceleration as they rotate, thereby producing a corresponding alternating output which is proportional to the rate of spin. Misalignment of the accelerometer sensitive axis from the rotor axis results in gravitational sensitivity if the rotor axis is not precisely vertical. While this can be overcome by accurate alignment and calibration, the slightest drift in alignment can cause overwhelming gravitational sensitivity. For example with a rotor speed of 50 Hz, an accelerometer positioned at a radius of 40 mm on the rotor, and the gyro compass tilted at up to 6 degrees, an alignment drift of only 10 micro-radians can induce an error of 0.5 degrees. As the gravitational signal has the same frequency as the Coriolis signal and indeterminate phase, separating these signals is problematic. As a result, a gravity-sensitive bias may exist in the outputs of these devices, to an extent that it is difficult to configure an accurate enough gyro-compass using this technology.